The present invention relates to a control system for a continuously variable transmission, and more particularly to a hydraulic control system having a hydraulic automatic clutch control.
In controlling a hydraulic automatic clutch of a vehicle, the clutch must be engaged smoothly for starting the vehicle. One of measures to meet this requirement is to increase a fluid pressure supplied to the hydraulic clutch in response to a revolution speed of an engine. This provides as smooth starting of the vehicle as the use of a centrifugal clutch does. However, the use of the fluid pressure variable as above poses a problem that the vehicle can not continue to travel during operation at engine speeds below a predetermined value because the clutch is released or put into a half engaged state whenever the engine speed drops below the predetermined value. This problem derives from the fact the fluid pressure variable with the engine revolution speed is used as the actuating pressure for the hydraulic clutch. For the purpose of better fuel economy, it is preferrable to operate a vehicle at lower engine revolution speeds and with a smaller reduction ratio. However, operating the vehicle in this manner is not feasible without solving the above mentioned problem.
In order to solve the problem above, the applicants have proposed in the co-pending U.S. patent application Ser. No. unknown concurrently filed and assigned herewith a control system for a hydraulic automatic clutch wherein a start adjustement valve supplies a starting valve with a start adjustment fluid pressure as applied against a fluid pressure indicative of a revolution speed of an engine so as to render the starting valve operable in a start fluid pressure regulating state. In the start fluid pressure regulating state, the starting valve generates a fluid pressure, which may be called as "a start fluid pressure," which is unchanged irrespective of a variation in idle speed of the engine. This fluid pressure is supplied to the hydraulic clutch so as to keep the clutch in a predetermined pre-engaged state. This proposed system also comprises a complete engagement control valve which selectively supplies a fluid pressure, which may be called as "a complete engagement fluid pressure," to the starting valve so as to render the starting valve operable in a complete engagement fluid pressure regulating state. When it is put into the complete engagement fluid pressure regulating state, the starting valve generates a fluid pressure higher than the start fluid pressure so as to assure complete engagement of the clutch even if the revolution speed of the engine drops.